Conventionally, as such a method for forming an anchor hole in which a diameter expansion portion is formed in apart of a prepared-hole portion, there has been known a method using an undercut drill device (see Patent Document 1). In this case, a resin-based anchor is anchored into the anchor hole, and the prepared hole is formed to have a relatively large diameter (a diameter of about 90 mm or more).
The undercut drill device has a hollow cylindrical barrel body inserted in the prepared hole, a putting member that is mounted on the opening edge of the prepared hole and rotatably supports the barrel body via a bearing, a shaft that slidably engages with the barrel body on the same axis and integrally rotates with the barrel body, a truncated-cone-shaped cone portion that is provided on the tip-end side of the barrel body and has four guide grooves on its outer peripheral surface, four arms that are attached to the tip end of the shaft and engage with the respective guide grooves, and two cutting blades and two guide portions alternately provided on the tip-end outer surfaces of the four arms.
The cutting blades and the guide portions are positioned inside the barrel body when the shaft is pulled up. When the barrel body and the shaft inserted in the prepared hole are integrally rotated and the shaft is moved downward, the four arms move downward and spread to an outside through the guide grooves of the cone portion. Thus, the cutting blades grind the inner peripheral surface of the prepared hole to form a diameter expansion portion at the bottom part (the deepest part) of the prepared hole.
[Patent Document 1] JP-A-2005-280243